Process for removing carbon dioxide from gas streams

ABSTRACT

A more efficient removal by selective adsorption of carbon dioxide from nitrogen-containing gas streams is achieved by using as the adsorbent a Type X zeolitic molecular sieve containing at least 90 equivalent per cent barium cations.

United States Patent i191 Sherman et al.

PROCESS FOR REMOVING CARBON -DIOXIDE FROM GAS STREAMS Inventors: John Delano Sherman, Chappaqua;

Ronald Jay Ross, Upper Nyack, both of NY.

Union Carbide Corporation, New York, NY.

Filed: Feb. 5, 1974 Appl. No.: 439,711

Assignee:

US. Cl 55/68; 55/75 Int. Cl B0ld 53/02 Field of Search 55/68, 75

References Cited UNITED STATES PATENTS 4/1959 Milton 55/75 X Primary Examiner.|ohn Adee Attorney, Agent, or Firm-Richard G. Miller [57] ABSTRACT A more efficient removal by selective adsorption of carbon dioxide from nitrogen-containing gas streams is achieved by using as the adsorbent a Type X zeolitic molecular sieve containing at least 90 equivalent per cent barium cations.

3 Claims, No Drawings PROCESS FOR REMOVING, CARB'ON' DIOXIDE FROM GAS STREAMS The present invention relates in general to the purification o'f gas streams by the removal of carbon dioxide as an impurity therefrom. and more particularly to a process for selectively adsorbing carbon dioxide from gas streams comprising nitrogen and carbon dioxide which utilizes as the adsorbent a barium cation form of zeolite X in which at least 90 percent of the framework AlO -tetrahedra of the zeolite lattice are associated with barium cations.

It is well known that carbon dioxide is readily adsorbed by the various cation forms of zeolite X. U.S. Pat. No. 2,882,244 in which zeolite X is described in detail and the method for its manufacture set forth. dis closes that at 25C and in a static system containing CO at a pressure of 750 mm. Hg, 26.3 weight per cent CO is adsorbed by the sodium cation form of zeolite X.

It is also known that both qualitative and quantitative changes in the cation population of the zeolite has a marked effect upon its adsorptive property with respect to a particular sorbate. The nature of the change is entirely unpredictable, however, as evidenced by the data in Table 1, below. taken from US. Pat. Nos. 2,882,244 and 3,078,639.

feed of these plants is ambient air supplied at a rate of 5000 to 5,000,000 SCFH, depending on the design capacity of the particular plant. It is particularly desirable to remove carbon dioxide and water from the feed gas before it enters the cold box in order to prevent freezeups in the heat exchangers and elsewhere in the system. It has now been found that carbon dioxide can be very efficiently removed from gas streams containing same at a concentration of not more than about 1000 ppm by volume by passing the said gas stream through a bed of activated zeolite X molecular sieve having a BaO/Al- Q molar ratio of at least 0.90 and, preferably, at least 0.95, at a temperature of from F to +120F, preferably less than +80F. at a pressure of from about 6 to 170 atmospheres, and at a feed rate to the bed of from 5000 to 5,000,000 standard cubic feet per hour. A complete description of zeolite X and the 'method for its manufacture is set forth in detail in US.

Pat. No. 2,882,244.

The gas streams to be suitably treated in accordance with the present process are air and mixtures comprising nitrogen and carbon dioxide, which may also contain additional oxygen and other gases inert toward the molecular sieve adsorbent and other components of the gas stream under the operating conditions employed. Argon, helium and neon can be present whether or not oxygen is also present but mixtures which contain hy- TABLE 1 Wt. 71 Adsorbcd by Adsorbate T. C P, mm Hg Na X CaX BaX MgX Argon l96 146 42.0 43.7 34.1 Nitrogen 75 500 9.6 7.9 10.0 -75 738 9.0 10.8 196 700 27.9 29.2 21.9 Water 25 4.5 29.3 33.3 25.8 34.2

24 35.5 39.5 27.7 n-hexane 25 20 19.2 15.8 18.3 Methyl Cyclohexane 25 38 20.7 16.8 21.4 Carbon Monoxide 75 500 15.8 10.0 75 25 5.4 8.1 0 25 2.5 0.8 0 750 5.1 5.1 6.8 Oxygen l83 700 35.6 37.6 27.7 Krypton -78 700 41.2 38.3 36.4 Xenon 78 700 72.6 71.0 59.0 Cyclohexane 25 19.6 16.8

It is apparent from the foregoing data that as a general rule, the barium-exchanged form of zeolite X has less capacity for a wide variety of sorbate molecules than does either the as-produccd sodium zeolite X or the calciumor magnesium-exchanged forms of zeolite X. The calciumand magnesium-exchanged forms,

moreover. exhibit a greater apparent capacity for certain substances than does the sodium form. and less ca- 'cularimportance is the low temperature separation ofair'by liquefaction. The

important considerdrogen or hydrocarbons should not also contain oxygen to avoid the hazards of combustion in the adsorbent bed. The presence of water vapor in the feed stream, however, reduces the capacity of the activated bariumexchanged zeolite X for carbon dioxide. Accordingly, in a typical process a feed stream such as ambient air is first suitably dried by heat-exchange to remove water by condensation, followed by contact with a separate, activated molecular sieve drying bed.

The proportions of the various components of the gas mixture are not at all critical, but since the process is designed primarily as a purification procedure rather thana bulk separation procedure. the carbon dioxide content of the gas stream should usually not be greater than about 1000 ppm by volume. The removal of carbon dioxide (approximately 300 to 400 ppm inlet concentration) from atmospheric air is a particularly advantageous embodiment of the present process. Another embodiment is the removal of very small amounts of carbon dioxide from nitrogen, such as postireatmcnt of nitrogen-containing streams which have indergone bulk CO- removal'. a typical example hereof is the removal of the small quantity of residual CO- in ammonia synthesis gas streams following treat- :rient by water-scrubbing. hot carbonate scrubbing. or .rthanolamine absorption.

although barium-exchanged forms of zeolite X have teen known for many years. the unique dynamic adiorption capacity of a highly exchanged material for run is terminated when the effluent CO concentration equals the influent CO concentration and has remained constant for one hour.

The barium-exchanged zeolite X employed in the experiments were prepared in the conventional manner using aqueous solutions of barium chloride at 95C as the ion-exchange medium. For the higher degrees of exchange the sodium zeolite X was subjected to several treatments with fresh ion-exchange medium.

TABLE 2 CO Adsorption Conditions Static. Wt.-7r Ads.

pellets from Sample No. 5, bariumexchanged ..tarbon dioxide was apparently not recognized in the JHOI' art. This unusual adsorption capacity is shown by the data set forth in Table 2 below. In determining the llaUC adsorption capacities of the adsorbents tested, the conventional McBain-Bakr system was employed. For the dynamic adsorption capacity determinations as eported in Table 2. the following apparatus and proceiure were utilized.

The apparatus for such determination has as its major .;omponents a Type 3A molecular sieve dryer. heat ex- .."nanger. adsorption column, temperature and pressure controllers. gas sampling system and CO analyzers. Temperature and pressure are held at 75 F; 1F and "5 psig. respectively. Flow rate of air supplied to the red is 150 or 100 SCFl-l. depending on the size of the adsorbent agglomerates under test. The bed weight is l to 4 pounds. The CO; analyzers are calibrated with Cog/N gas mixtures of known compositions. Approxiin tablet or pellet form are activated under vacuum for .it least 16 hours at 350C. The final activation pressure lhOUld be 100 microns Hg. preferably less than 50. or even microns. The vacuum flasks are removed from he system and allowed to cool to room temperature. The vacuum is then broken with dry N The sample is charged into the column while the column is being .;oun tercurrently purged with N The column is presiUYlZCd with land the bed is allowed to cool to 6F. At that time thermocouples for the influent and effluent gas should read the same temperature on the recorder iwithin I 11 T. The N flowing through the :olumn is shut off and the air is turned on. bypassing the column. The experimental flow rate is then set and he gas flow rate (2 SCFH) to the CO analyzer is also idiu sted. Once this is accomplished. the air is directed through the column and the experiment is begun. The

Referring to Table 2. it is seen that sodium zeolite X powder, exchanged with barium cation to a value of 0.95 molar BaO/Al O (Sample No. 1) had an unexpectedly high CO sorption capacity of 17.7 wt.-% measured under equilibrium conditions (25 Torr) in the McBain-Bakr apparatus. This is in marked contrast to the sorption capacity of a typical sodium X powder under the same adsorption conditions (Sample No. 2).

1n the dynamic capacity determinations, Sample No. 3 (tableted form of Sample No. 1) exhibited a value of 6.0 wt.-% CO which is over higher than the value of 3.8 Wt.-% CO for Sample No. 4, also in tablet form. Comparing the dynamic adsorption data for Samples No. 5 and 6, both with clay binder, it is seen that barium exchange again gave improved performance; even higher capacity would have been achieved if the degree of exchange in Sample No. 6 had been increased beyond the value of 0.83 BaO/Al O mately 16001800 grams of the molecular sieve sample 50 2 3 EXAMPLE The following illustrates using this novel adsorbent in a dual-bed cyclic prepurification system according to 5g the method of the invention:

A plant for the separation of air by liquefaction requires 2.050.000 SCFl-l of atmospheric air feed at 40F and 600 psia. This feed is water-saturated and its CO content is 320 ppm (by volume). Two dual-bed molec- 0 ular sieve adsorber units are employed in series arthe purifier. is for removal of CO and likewise is operated in a cyclic manner.

The drier unit treats this feed stream so that a dew point of-l00F is achieved. The dried stream is passed to the purifier unit operated at 40F and 600 psia. Each adsorber bed of this purifier unit is charged with l0.800 pounds of 0.95 (BaO/Al O molar ratio) Ba-exchanged zeolite X in the form of l/l6-inch pellets. The concentration of CO in the purifier effluent stream being supplied to the air plant heat exchangers is maintained at less than one ppm (vol) during each'R-hour adsorption period. I

The foregoing Weight of adsorbent charged is to be compared with the conventional charge of 18.400 pounds of pellets of the sodium cation form of zeolite X in each bed of this unit.

What is claimed is:

1. Process for removing carbon dioxide from gas streams which comprises providing a gas stream comprising nitrogen and carbon dioxide, said gas stream containing not more than about 1000 ppm by volume of carbon dioxide, passing said gas stream through an adsorption zone at a temperature of from -40F to l20F at a pressure of from about 6 to atmospheres and at a feed rate of from 5000 to 5.000.000 standard cubic feet per hour. said adsorption zone containing the barium cation form of zeolite X in which the BaO- /AI O molar ratio is at least 0.90. whereby the carbon dioxide is selectively adsorbed and the effluent from the adsorption zone is substantially free of carbon dioxide.

2. Process according to claim 1 wherein the gas stream being treated is air.

3. Process according to claim 2 wherein the barium cation form of zeolite X has a molar BaO/Al- O ratio of at least 0.95. 

1. PROCESS FOR REMOVING CARBON DIOXIDE FROM GAS STREAMS WHICH COMPRISES PROVIDING A GAS STREAM COMPRISING NITROGEN AND CARBON DIOXIDE, SAID GAS STREAM CONTAINING NOT MORE THAN ABOUT 1000 PPM BT VOLUME OF CARBON DIOXIDE, PASSING SAID GAS STREAM THROUGH AN ADSORPTION ZONE AT A TEMPERATURE OF FROM -40*F TO + 120*F AT A PRESSURE OF FROM ABOUT 6 TO 170 ATMOSPHERES AND AT A FEED RATE OF FROM 5000 TO 5,000,000 STANDARD CUBIC FEET PER HOUR, SAID ADSORPTION ZONE CONTAINING THE BARIUM CATION FORM OF ZEOLITE X IN WHICH THE BAO/AL2O3 MOLAR RATION IS AT LEAST 0.90, WHEREBY THE CARBON DIOXIDE IS SELECTIVELY ADSORBED AND THE EFFLUENT FROM THE ADSORPTION ZONE IS SUBSTANTIALLY FREE OF CARBON DIOXIDE.
 2. Process according to claim 1 wherein the gas stream being treated is air.
 3. Process according to claim 2 wherein the barium cation form of zeolite X has a molar BaO/Al2O3 ratio of at least 0.95. 